Loop frames on a switch

[MATTHIAS SCHAERER]

What is the purpose of a loop frame on a switch interface?

[pradeepde]

Frames will loop forever (there is no time-to-live field in LAN frames) if there are multiple distinct paths between any two bridges.

To handle arbitrary topologies, bridges use a special protocol to build a spanning tree:

* Bridges that are not part of the spanning tree are unused. That is, they are specifically excluded from the tree and do not forward packets. They are available for backup, however, should one of the other bridges or LANs fail.

* Bridges periodically rebuild tables. They regularly exchange topology information, allowing them to detect the failure of a bridge or LAN. When a bridge or link that is part of the spanning fails, a new spanning tree is constructed.

[Richard Burts]

I think that the original post was referring to an Ethernet frame in which the source MAC address is the switch port's MAC address (as expected) and the destination address is also the switch port MAC address. Since the source and destination are the same address some peoople refer to it as a loop frame.

This is the standard implementation of an Ethernet Keepalive frame.

HTH

Rick